The invention concerns improvements made at the time of manufacturing of containers of thermoplastic material such as bottles, bowls or any other type of containers by blow molding in the finishing molds, of blanks having previously been extruded or injection molded or even consisting of intermediate containers obtained at the time of a prior blow molding step.
Various procedures and systems for manufacturing containers by blow molding blanks of thermoplastic material are known.
In the systems called extrusion-blow molding, the blanks called preforms, are obtained by extrusion of the material. For this purpose, an extrusion device is connected to the system, which makes it possible to obtain a tube either sequentially or continuously. This tube is extruded and brought by gravity or by guiding to the interior of the finishing molds.
Then a determined length of the tube is held in the finishing molds; next a gas under pressure, generally air, is blown into the part that is held, making is possible to expand it and to give it the shape of the cavities of the mold and resulting in the desired containers.
In the systems called injection-blow molding, the blanks, called preforms, are obtained by injection of the material into a connected device on, or separate from, the system. The preforms are in the shape of tubes open at one end and closed at the other. The preforms are introduced into the finishing mold cavities as soon as they are at a temperature that is adequate for blow molding them. Thus, when the preforms are injected in a device that is separate from the blow molding system, the latter has a reheating furnace for the preforms to soften them and permit blow molding of the containers.
In addition, there are systems using successive blow molding steps: an intermediate container is formed in a first mold starting with a blank, then it is blow molded again in a second mold. Thus, the intermediate container is a blank with respect to the final container.
Among the known systems, no matter whether it concerns extrusion-blow molding systems or injection-blow molding systems, there are those that have at least two molds which are moved by a rotating movement in the system in such a manner as to deliver the blanks successively into the molds in a loading zone, to an intermediate zone in which, among other things, the blow molding of the containers takes place and the completed containers are brought into an ejection zone.
Systems such as this can have high production rates since when a mold is being filled, another may be in the process of blow molding and another may be in the process of ejection.
In addition, the system needs only one device for loading and one device for ejection. Also systems such as this make it possible for the manufacturing process to be reproducible and avoid production scatter.
Naturally, it is understood that the higher the number of molds, the more the rates may be elevated.
Thus, by way of example, in the line of machines produced by the applicant, there are injection-blow molding machines and extrusion-blow molding machines in which the molds are carried by a carousel that is moved by a continuous rotation movement. The presence of this carousel makes rapid rotation possible; in addition, this rotating machine technology makes it possible to ensure excellent synchronizing of the operations among the various elements of each machine set up in this way.
In the sense of the present invention, mold must be understood as a subassembly component having one or several cavities. The cavity or cavities have the external shape of the container to be manufactured. The French patent request published under the number 2 709 264, in the name of the applicant, describes an injection-blow molding machine in which each mold has at least two cavities (or imprints of the containers).
Blank is intended to mean preform or intermediate container, i.e., most generally any element that is able to be transformed into a container (itself being intermediate or final) by blow molding.
In a known manner, the manufacturing of a container as well as the loading of the blank and the ejection of the completed container requires at least one blow molding with gas, generally air, at elevated pressure and gas removal before ejection.
In certain improved implementations, a supplementary flow of gas under pressure is injected after partial or total gas removal before the intermediate or final container can be removed from its blow molding mold. The injection of gas is generally carried out at a pressure that is lower than the blow molding pressure. This supplementary injection is carried out particularly in procedures that use blow molding by thermofixation, i.e., the procedures in which the containers are blow molded in very hot molds. It has been confirmed that, in a surprising manner, this supplementary injection makes it possible to obtain containers that have mechanical resistance or other elevated properties when they are used.
Generally, the supplementary injection consists of a sweeping rather than a reblowing, i.e., of a circulation of gas in the container. For this reason, during the sweeping, the container is open at least partially to the outside air in order to permit this circulation.
One disadvantage of the known systems is that they are very high consumers of blow molding gas, a problem that is further increased when supplementary injection is used. Thus, for example, in the injection-blow molding systems produced by the applicant, the blow molding air is applied at a pressure of 40 bars. This indicates that the blow molding of a one-liter container requires forty liters of air, that of a two-liter container requires eighty liters, etc.
The supplementary injection carried out between 10 and 20 bars thus requires between 10 and 20 liters of air for a one-liter container, 20 to 40 liters for a 2-liter container. Still it is necessary to mention that the applicant""s machines have a rate between approximately 1,200 and 50,000 containers per hour, which corresponds to a theoretical consumption of air under pressure between 66,000 and 2,750,000 liters of air per hour to manufacture one-liter containers with a supplementary injection at 15 bars.
The goal of the invention is a procedure in which the consumption of compressed gas is reduced.
According to the invention, a procedure for manufacturing containers by blow molding using gas at a first pressure (P1), in particular air, of blanks on the interior of molds in a system containing at least two molds in which the blanks are successively introduced and the containers are successively blow molded, the forming of each container comprising, after a blow molding step, a step for at least partial degassing following a step in the course of which gas at a second pressure (P2) is introduced into the container and is characterized in that the gas at the second pressure (P2) is made up of at least part of the product of degassing of at least one container that is blow molded further on in the system.
Thus, the consumption of gas under pressure is considerably reduced. In addition, given that the supplementary injection of gas is carried out by transfer, it is not necessary to provide means for producing supplementary gas under pressure: it is sufficient simply to provide the adapted interconnections between the different molds. Because of this, the system is simplified and its cost is considerably reduced.
In contrast, and this is fundamental, the performance of the system is not reduced in any way. In fact, as indicated above by way of example, in the applicant""s machines, the supplementary injection is carried out with gas at a pressure between 10 and 20 bars and initial blow molding is carried out at 40 bars. Assuming that the supplementary injection consists of reblowing at a lower pressure, it is possible, starting with a container that is blow molded at 40 bars, to degasses it into another of the same volume which has already been degassed in order to obtain a residual pressure of 20 bars in each of the containers (application of Mariotte""s Law).
If, as is usually the case, the supplementary injection consists of a sweeping of the container, i.e., a circulation of gas in the container permitting the gas to escape into the outside air during this operation, then the sweeping pressure can be decreased from 40 bars to zero during the sweeping if the following container is completely degassed in order to carry out the sweeping of the preceding one.
According to another characteristic, the supplementary injection carried out in a container is carried out using the blow molded container in the mold that follows immediately in the system.
Thus, the pneumatic connections are easy to implement.
In one embodiment, when the system has at least three molds, the supplementary injection is carried out by causing an introduction in cascade of one part of the product of degassing of several containers into another.
This creates a turbulent movement which again improves the efficiency of the sweeping.
According to another characteristic, in one embodiment, each mold has at least two blow molding cavities, the product of degassing of one container manufactured in one cavity of a mold is transferred into a corresponding cavity of a preceding mold.
This makes it possible to facilitate the system of pneumatic connections while avoiding crossing of the tubes.